The discovery of photoinduced electron transfer between conjugated polymers and fullerene (C60) (Sariciftci, Wudl et al. 1992) and the optimized photovoltaic devices (˜6.0% power conversion efficiency) fabricated from regioregular poly(3-hexylthiophene)s (rrP3HTs) and soluble derivative of fullerene (PCBM) (Kim, Carroll et al. 2007), opened new ways for the realization of flexible photovoltaics based on semiconducting polymers (Seyte, Camps et al. 1985; Savechika, Yamamoto et al. 1985). The demand for further optimization of the photovoltaic efficiency has stimulated an intensive research effort both for new low band gap polymeric materials, acting as electron donors, but also for new efficient electron accepting materials.
In the last case, carbon nanotubes (CNTs) have gained interest as an alternative to the fullerene derivatives, because of their dimensions which enable the transport of the charge along the axis of the nanometers-long carbon structure and can contribute to a reduced probability of back transfer to the oxidized electron donor polymer. The first report of a polymer:single wall carbon nanotube (SWNT) photovoltaic device was in 2002 (Kymakis and Amaratunga 2002), utilizing composites of SWNTs with regioregular poly(3-octylthiophene) (rrP3OT). Ever since, many efforts have been presented towards the fabrication of CNTs-based photovoltaic devices (Guldi, Ford et al. 2005; Sgobba, Prato et al. 2006). However, limited power conversion efficiencies were found. Thus, several obstacles need to be properly addressed, such as the processability of the nanotubes and the selection of a proper set of electron donors to facilitate higher power conversion efficiencies from such photovoltaic devices.
The initial disadvantage of poor solubility has been partially overcome by various functionalization techniques (Tasis, Prato et al. 2006) using either the covalent attachment (Chen, Haddon et al. 1998) of different organic groups through reactions onto the 7-conjugated backbone of the SWNTs or the noncovalent absorption (Chen, Walker et al. 2002) or wrapping of various functional polymers (Richard, Mioskowski et al. 2003). Grafting to (Mansky, Hawker et al. 1997) or grafting from (Zhao and Brittain 2000) techniques have also been used for the conventional polymer modification of nanotubes. In order to prevent the 1-conjugation disruption, various modifications (Campidelli, Prato et al. 2006) that allow the weak functionalization of the SWNTs resulting in the lowest possible distortion of their electronic properties, can be used.